U.S. Pat. No. 4,761,979, which was granted to Mitsubishi Denki of Tokyo (Japan) describes a roller bending apparatus of a pyramidal kind, having a curvature measuring unit adapted to measure a radius of curvature of a workpiece comprising at least three probes mounted on a probe holder, at least one of the probes being a moving probe whose linear displacement produces a corresponding electrical output signal, with the rest of the probes being stationary probes. A drive cylinder of the curvature measuring unit moves the probe holder towards the workpiece until all of the probes firmly contact the surface of the workpiece, in which state the electrical output signal is applied to a calculating and display unit as a measured value, that is provided as an input value by an operator to a computer. The computer calculates a stroke measure of a machine top roller that is requested to obtain a desired curvature radius of the workpiece.
Further, the European patent N. 477 752, that was granted to Promau s.r.l. of Cesena (Italy) describes a roll bending machine for bending iron sheets that uses a three-point mechanical sensing device for the workpiece exiting the roll bending machine. This device enables the radius that the machine is conferring to the iron sheet to be checked, allowing one person to intervene for making necessary corrections.
European patent application EP 1 644 140 in the name of ORTIC AB of Borlange (Sweden) describes a method of process monitoring and controlling for continuous bending of an elongated workpiece to a predetermined radius by using three parallel contact-free distance meters of the laser transmitter type and by measuring the distances to the bent surface on the elongated workpiece, by calculating the actual curvature radius based on the fixed distances between the meters and the measured distances, and adjusting the bending machine in response to the relation between the calculated actual radius and the desired radius.
It is clear that in the above cited documents the actual radius of a section of an elongated workpiece is measured but the machine is corrected or adjusted while a workpiece section different from the measured one is bent. However, if the bend that one wants to obtain is a predetermined fixed radius bend, the method can be satisfying as it can imply that only a first bend section of the workpiece is of an actual bend radius different from the desired one. In this case, the damage can consist of throwing away that first bend section.
On the contrary, when one wants to bend an elongated workpiece at variable curvature radii, for example sections of a workpiece with a fixed radius that are separated by different radius connections or, generally, bend sections having a continuously varying radius, it is more advantageous to measure a radius of the elongated workpiece section that has a curvature radius near the radius of the bend section that the machine is working in that moment or straight after.
Consequently, a main object of the invention is to measure a curvature radius in a bend section that is not the actual curvature radius of a real bend section that is already formed, but that is the curvature radius of a bend section being formed by the machine.
Another object of the invention is to measure a curvature radius in a point nearest to the point of bending deformation by the machine, that is generally the third roller for the workpiece exiting the machine.